1. Field of the Invention
The present invention relates generally to a display apparatus for simultaneously displaying multiple pieces of information on mobile units (vehicles), such as their positions. More particularly, the invention pertains to a vehicle information display apparatus for displaying necessary pieces of information on vehicles with high visibility. The following discussion deals with a case where such a vehicle information display apparatus is installed on a vessel.
2. Description of the Prior Art
Installation of a system named a universal shipborne automatic identification system (AIS) has been compulsorily required on large vessels from July 2002 to assist in avoiding collisions between the vessels and thereby promote the safety of life at sea.
An important function of the AIS is to broadcast at specific intervals various items of information, such as the position of own ship (prime vehicle), navigational information including voyage information, as well as vessel-related information including vessel name and cargo type. Another important function of the AIS is to receive such information broadcast from other vessels and extract needed pieces of information.
Carriage of an automatic radar plotting aid (ARPA), on the other hand, which is already used on many vessels to aid in avoiding collisions with other target vessels (target vehicles) by use of a radar image, is mandatory on large vessels. Upon receiving target echo data from a radar, the ARPA processes the target data, automatically acquires and tracks target vehicles, calculates the degree of risk of collisions with the target vehicles, and presents target information in readily recognizable form, allowing an operator to continuously watch the movements of target vehicles. When the ARPA detects a dangerous situation, it generates a warning and enables the operator to execute a simulated “trial maneuver” to find out an own ship maneuver for avoiding the collision situation.
More specifically, the ARPA determines successive target positions from the target echo data fed from the radar, calculates relative motion (relative course and speed) of a target vehicle relative to the prime vehicle from varying relative positions of the target vehicle, and work out true motion (two-dimensional true course and speed) of the target vehicle taking into account of the velocity (vector quantity) of the prime vehicle.
An on-screen presentation of the ARPA is produced by superimposing various markings (symbols) representing target information, such as the present and predicted (calculated) positions and motions of the target vehicles, on a radar image.
It is preferable for the operator of the ARPA and the aforementioned AIS systems that any targets acquired and tracked by the ARPA (hereinafter referred to as the ARPA targets) and vessels identified by the AIS (hereinafter referred to as AIS targets) be presented on a common display in graphical form.
As stated above, installation of the AIS system has been mandated since July 2002 on a specific class of large vessels. Since this mandatory carriage requirement of the AIS system is initially limited to the large vessels, target vehicles detected by the AIS (hereinafter referred to as the AIS targets) are all large vessels at the beginning. The mandatory carriage requirement is expected to be applied to smaller vessels in the future, however. Therefore, if symbols of all the AIS targets identified by the AIS system are to be displayed on a single display, the number of target symbols displayed could be considerably large in a future time, particularly in congested areas, such as in navigable waterways and harbor areas, making it difficult to identify the individual AIS targets. Similarly, the number of symbols representing target vehicles detected by the ARPA system (hereinafter referred to as the ARPA targets) could be too large in congested areas, making it difficult to identify the individual ARPA targets.
Shown in FIG. 3C is an example of an AIS display presenting AIS targets all together, in which small squares are symbols indicating the positions of the individual AIS targets, small inverted triangles are symbols indicating the positions of individual ARPA targets, and broken lines indicate a simplified radar image of a coastline. This example shows a situation in which a large number of vessels are navigating close to the coastline of a strait. If cross-channel ferries, workboats and fishing vessels are present in such a water area, the AIS display would actually provide this kind of intricate picture.
Both the ARPA and the AIS are originally intended to enhance the safety of navigation. One problem of the conventional systems is that if all targets including those which may be ignored from the viewpoint of navigation safety are displayed, it would become difficult for an operator of a target information display system to identify important targets on which the operator should primarily focus particular attention for safe navigation.
As previously mentioned, the ARPA targets and the AIS targets are presented on a common display screen in graphical form. As a consequence, a symbol indicating the position, speed and course of an ARPA target derived from a particular target vehicle and a symbol indicating the position, speed and course of an AIS target derived from the same target vehicle are indicated at approximately the same location on the display screen, overlapping one on top of another. This overlapping of the symbols makes it difficult to recognize the position, speed and course of each target.
In addition, if part of acquisition and tracking capacity of the ARPA is used for acquiring and tracking ship targets which have already detected by the AIS system, that part of the limited capacity of the ARPA would be consumed uselessly, resulting in a reduction in the number of targets that can be acquired and tracked by the ARPA.